Crystalline bipyridinium radical complexes and uses thereof

ABSTRACT

Described herein are methods of generating 4,4′-bipyridinium radical cations (BIPY •+ ), and methods for utilizing the radical-radical interactions between two or more BIPY •+  radical cations that ensue for the creation of novel materials for applications in nanotechnology. Synthetic methodologies, crystallographic engineering techniques, methods of physical characterization, and end uses are described.

CROSS-REFERENCE TO RELATED APPLICATIONS

The benefit under 35 U.S.C. §119 of U.S. Provisional Application No.61/537,852, filed Sep. 22, 2011 is claimed, the disclosure of which isincorporated by reference in its entirety.

STATEMENT OF GOVERNMENT SUPPORT

This invention was made with U.S. government support under Grant No.DMR-0520513 awarded by the National Science Foundation; Grant No.FA9550-07-1-0534 awarded by the Air Force Office of Scientific Research(AFOSR); Grant No. W911NF-10-1-0510 awarded by the Army Research Office(ARO); and Grant No. DE-SC0005462 awarded by the Department of Energy(DOE). The government has certain rights in the invention.

FIELD OF THE INVENTION

The invention relates to molecular compounds and complexes incorporatingBIPY^(•+) derivatives that are made to interact by means ofradical-radical interactions with the diradical dicationiccyclobis(paraquat-p-phenylene) (CBPQT^(2(•+))) ring. Host-guestinclusion complexes and mechanically interlocked molecules (MIMs) aredescribed.

BACKGROUND

The ability of BIPY^(•+) radical cations to exist as stable radicalcations has long been known, and their tendency to undergo dimerization[(BIPY^(•+))₂] by means of favorable radical-radical interactions (alsoreferred to as pimerization) well is documented. However, thestabilities of these BIPY^(•+) radical cation dimers are rather weak,especially in organic solvents, which challenges their use inapplications.

SUMMARY

Applicants have discovered a means to increase the stabilities ofradical-radical BIPY^(•+) interactions by use of host-guest chemistryutilizing the CBPQT^(2(•+)) ring. This has allowed for the furtherdevelopment of template-directed synthetic methodologies in theconstruction of MIMs, and the ability to mechanically switch these MIMsby transfer of electrons. Applicants have also devised crystalengineering techniques for the production of novel solid-state materialscomposed of these MIMs and host-guest complexes, while in their radicalcationic forms. Throughout this disclosure the BIPY guest molecule isalternatively referred to as a compound of formula (I).

In some embodiments the BIPY^(•+) guests have been alkyl derivatives,for example

In some embodiments the BIPY^(•+) guests have been aryl derivatives, forexample

By one method, complexation of BIPY^(•+) guests with the CBPQT^(2(•+))ring can be induced light using the Ru(bpy)₃ ²⁺ photosensitizer.

By one method, complexation of BIPY^(•+) guests with the CBPQT^(2(•+))ring can be induced by chemical reduction using zinc dust.

By one method, complexation of BIPY^(•+) guests with the CBPQT^(2(•+))ring can be induced electrochemically at the surface of an electrode.

In some MIMs with BIPY^(•+) and CBPQT^(2(•+)) ring components, switchinghas been induced by light using the Ru(bpy)₃ ²⁺ photosensitizer.

In some MIMs with BIPY^(•+) and CBPQT^(2(•+)) ring components, switchinghas been induced chemically using zinc dust as a reducing agent.

Complexation can be performed in either water or organic solvent.

The synthesis of MIMs can be achieved using athreading-followed-by-stoppering approach.

The synthesis of MIMs can be achieved using a clipping approach.

Crystallization of complexes can be achieved using slow-vapor diffusionin MeCN with iPr₂O as the bad solvent under an inert atmosphere.

Crystallization of MIMs can be achieved using slow-vapor diffusion inMeCN with iPr₂O as the bad solvent under an inert atmosphere.

Crystallization can be carried out at concentrations ranging from 10 mMto 40 μM of the mother liquor.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1. Simplified schematic representation using structural formulas ofthe redox-induced formation of the trisradical cationic inclusioncomplex CBPQT^(2(•+))⊂MV^(•+) and its graphical representation. Upon athree-electron reduction of an equimolar mixture of CBPQT⁴⁺ and MV²⁺,the MV^(•+) radical cation is included spontaneously inside the cavityof the CBPQT^(2(•+)) diradical dication ring as a result of favorableradical-radical interactions occurring between the three BIPY^(•+)radical cations. Re-oxidation of the tricationic complex results in theregeneration of the initial uncomplexed CBPQT⁴⁺ host and MV²⁺ guest.

FIG. 2. a) Cyclic voltammogram of an equimolar mixture of CBPQT⁴⁺ andMV²⁺ in MeCN at 1.0 mM. The first reduction process is a three-electronone leading to the formation of the trisradical CBPQT^(2(•+))⊂MV^(•+)inclusion complex. As a consequence of radical pairing between oneBIPY^(•+) of the ring and that from MV^(•+), the reduction of thecomplex is observed to occur over, first by a one-electron process,followed by a two-electron one the former (and less negative) one isassigned to the reduction of the unpaired BIPY^(•+) unit in the ring. b)Cyclic voltammogram of the [2]rotaxane R⁶⁺ containing only a BIPY²⁺recognition unit in its dumbbell component. As a consequence of bothradical pairing and the mechanical bond, the first reduction processesleading to the trisradical species R^(3(•+)) is observed to occur over,first by a two-electron process, followed by a one-electron one. Thefirst two-electron process is assigned to the simultaneous reduction ofone BIPY²⁺ unit of the ring and the BIPY²⁺ unit of the dumbbellcomponent, leading to the formation of a stable bisradical intermediate.Both voltammograms were recorded at a scan rate of 200 mV s⁻¹.

FIG. 3. a) A CV titration of a solution of MV²⁺ (10 mM) into a solutionof CBPQT⁴⁺ (1.0 mM) in MeCN at 298 K. Blue trace: CBPQT⁴⁺ only. Purpletrace: 1:1 mixture of CBPQT⁴⁺ and MV²⁺. b) Variable scan rate CVs of the1:1 mixture of CBPQT⁴⁺ and MV²⁺ in MeCN at 298 K. Blue trace: 200 mVs⁻¹; red trace: 1000 mV s⁻¹; pink trace: 30 V s⁻¹.

FIG. 4. Experimental and simulated variable scan rate cyclicvoltammograms of a 1:1 mixture of CBPQT⁴⁺ and V²⁺ in MeCN at 298 K. As aconsequence of radical pairing, one of the BIPY^(•+) radical cationunits is oxidized first (at a less negative potential) during the scan,leading to the formation of the bisradical CBPQT^((2+)(•+))⊂V^(•+)intermediate. The presence of the BIPY²⁺ unit in the CBPQT^((2+)(•+))ring leads to dissociation of the complex observable on the timescale ofthe CV experiments. Fitting the experimental data to the digitalsimulations, the free energy barrier ΔG^(‡) to dissociation of thebisradical CBPQT^((2+)(•+))⊂V^(•+) complex was determined to be 16.0kcal mol⁻¹. Blue trace=200 mV s⁻¹; purple trace=10 V⁻¹. IR compensationwas applied.

FIG. 5. Variable scan rate cyclic voltammetry of the [2]rotaxane R⁶⁺obtained in MeCN at 298 K with 0.1 M TBA.PF₆ as the supportingelectrolyte. Blue trace: 50 mV s⁻¹; red trace: 100 mV s⁻¹; black trace:200 mV s⁻¹; green trace: 600 mV s⁻¹; pink trace: 1000 mV s⁻¹.

FIG. 6. Relative integration analysis of the dissociation of thebisradical complex CBPQT^((2+)(•+))⊂V^(•+) into its free componentsbased on variable scan rate CV data shown in FIG. 4.

FIG. 7. a) Variable scan rate CV of the DNP-BIPY²⁺ [2]rotaxane R1⁶⁺. Theblue trace was taken at a scan rate of 10 mV s⁻¹ leading up tosuccessively higher scan rates up to 500 mV s⁻¹, shown as the red trace.The re-oxidation peak at 0 V increases in relative intensity with fasterscan rates. b) Quantifying the R-MSCC versus time from the variable scanrate data. Fitting the amount of R-MSCC versus time to a first orderdecay profile allows for the determination of a barrier to relaxation of19.1 kcal mol⁻¹. c) A thermodynamic landscape constructed from thesedata for the bisradical oxidation state depicting the relaxation of theR-MSCC to the R-GSCC.

FIG. 8. Solid-state superstructures of the trisradical cationCBPQT^(2(•+))⊂MV^(•+) inclusion complex obtained by single-crystal X-raycrystallography. a) Wireframe representation of theCBPQT^(2(•+))⊂MV^(•+) inclusion complex. b) Ball-and-stickrepresentation of the CBPQT^(2(•+))⊂MV^(•+) inclusion complex from aside-on perspective, illustrating the angle of offset from orthogonalityof the MV^(•+) occupying the cavity of the CBPQT^(2(•+)). c) Plan viewusing a space-filling representation of the long-range packing order ofthe trisradical tricationic CBPQT^(2(•+))⊂MV^(•+) inclusion complex,which forms a continuous radical-radical π-stack. The PF₆ ⁻ counterions,have been omitted for clarity. d) Side-on view showing the relativepositions of the PF₆ ⁻ counterions.

FIG. 9. Solid-state superstructure of the diradical dicationCBPQT^(2(•+)) ring obtained by single crystal X-ray crystallography. a)In common with the trisradical CBPQT^(2(•+))⊂MV^(•+) inclusion complex,the CBPQT^(2(•+)) solid-state superstructure reveals a CBPQT^(2(•+))rings of radical cation-radical cation along the a crystallographic axisinteractions between its BIPY^(•+) constituent subunits. Thecounterions, PF₆ ⁻ have been omitted for clarity. b) A wireframerepresentation (plan view) of the solid-state superstructure ofCBPQT^(2(•+)), depicting the porous channels formed by the unoccupiedinteriors diradical dicationic rings.

FIG. 10. The isothermal titration calorimetry (ITC) traces obtained whenmeasuring the thermodynamic parameters (K_(a), ΔH, ΔS) governing theformation of the trisradical CBPQT^(2(•+))⊂MV^(•+) inclusion complex.All the ITC experiments were performed in MeCN at 298 K. The values forK_(a), ΔH and ΔS reported above are averages over four runs, and theerrors are the associated standard deviations from these four runs.

FIG. 11. Electronic spectra of CBPQT^(2(•+)) and MV^(•+). Comparison ofthe electronic spectrum of CBPQT^(2(+•)) with the sum of the electronicspectra of two MV^(•+) indicates significant differences in agreementwith strong electronic coupling in the ground state between the twochromophores. Solvent: MeCN; T=25.0(2) ° C.

FIG. 12. UV/Vis Absorption spectrophotometric titration of CBPQT^(2(•+))by MV^(•+). Solvent: MeCN; T=25.0(2) ° C.; Reducing agent: activated Zndust; l=1 cm. (1) [CBPQT^(2+•)]₀=4.36×10⁻⁵ M; (2)[MV^(+•)]₀/[CBPQT^(2+•)]₀=4.38.

FIG. 13. The radically promoted, template-directed synthesis of the[2]rotaxane R.6PF₆ via the intermediacy of the highly stable[2]pseudorotaxane [T^(•+)⊂CBPQT^(2(•+))] which affords the [2]rotaxaneR^(3(•+)), prior to its subsequent aerial oxidation to give R.6PF₆during its purification and isolation. The PF₆ ⁻ counterions are omittedfor the sake of clarity. The graphical representations of the structuralformulas have been introduced to aid and abet the presentations ofmolecular structures in FIGS. 2-4.

FIG. 14. Template-directed synthesis of the homo[2]catenane C^(4(•+))employing a clipping strategy relying on BIPY^(•+) radical-radicalinteractions. The synthesis begins with a 1:1 mixture of CBPQT⁴⁺ andVB²⁺ in MeCN. The solution is exposed to zinc dust in order to bringabout reduction of the components to their radical cation forms andinduce complexation. The zinc dust is then filtered off and4,4′-bipyridine is added to the solution. A trisradical intermediatecomplex undergoes cyclization to afford the final interlocked product.

FIG. 15. Diagram of the Organic Field Effect Transistor (OFET) used tostudy the conductivity of the nanowire crystals ofCBPQT^(2(•+))⊂MV^(•+). Conductivity of a single crystal is measured as afunction of the gate voltage. Such measurements allow for thedetermination of n- or p-type semiconductor characteristics.

FIG. 16. Field effect transistor characteristics of the OFET composed ofsingle crystals of CBPQT^(2(•+))⊂MV^(•+) crystals grown on a SiO₂/Sisubstrate: a) output characteristic of the OFET device and b) transfercharacteristics of the OFET device, showing p-type semiconductorbehavior.

FIG. 17. Proposed single-layer organic solar cell (SLOSC) employingsingle crystals of CBPQT^(2(•+))⊂MV^(•+) as the photoactive component.a) Construction of the devices begins with a layer of (1.) aluminumserving as the hole-collecting electrode. Growth of theCBPQT^(2(•+))⊂MV^(•+) crystals in the presence of a magnetic field (B)is expected (2.) to result in crystals all aligned in a similardirection. Deposition of a layer of indium tin oxide (ITO) as theelectron-collecting electrode (3.) and a protecting glass substratecompletes the construction of the device. b) Upon irradiation of light,a photocurrent is expected to result. The high level of order in thedevice is excepted to result in efficient excitation separation leadingto high efficiency.

FIG. 18 shows three examples of nanowires formed from crystal complexesdisclosed herein.

DETAILED DESCRIPTION

Disclosed herein are complexes of CBPQT^(2(•+)) and a guest molecule,e.g., a compound of formula (I):

R¹ and R² are each independently selected from alkyl, cycloalkyl,heterocycloalkyl, aryl, heteroaryl, alkenyl, alkynyl, alkyleneazido,alkylenecycloalkyl, alkyleneheterocycloalkyl, or alkylenearyl. Each R¹and R² can be the same or different. Choice of steric and electronicproperties of the R groups can influence the end properties of thecomplex. For example, the complex can be tuned by these selections toprovide a complex with a particular conductance, redox potential, and/orUV-vis property.

These complexes can be isolated as crystals, e.g., single crystal,structures. The complex also has a single unpaired electron which canallow for control of crystal growth of the complex by application of anexternal magnetic field. This single unpaired electron is stable becauseit is delocalized over the whole complex. Since host-guest chemistry isused prior to crystal growth, it allows for creation of a large varietyof materials by inserting different guests, with different electronicproperties into the host. In this way the electronic properties of thematerial (crystal) can be tuned in a very modular way.

Synthesis of Compounds of Formula (I)

Symmetric 1,1′-dialkyl-4,4′-bipyridinium derivatives can be synthesizedin one step starting from 4,4′-bipyridine and an alkyl derivativefunctionalized with a good leaving group (X), e.g., chloro, bromo, iodo,tosyl, mesyl, etc. In general, the syntheses are performed in refluxingacetonitrile in the presence of two or more equivalents of the alkylderivative functionalized with the good leaving group. Isolation of thetarget does not require column chromatography in most cases. After thereaction has come to completion, the acetonitrile can be removed to aminimal volume, and the residual solid is converted to its PF₆ ⁻ salt byaddition of an aqueous solution of NH₄PF₆. The resulting water insolublesalt is filtered, washed with water leaving behind the target product inpure form and ready to use in the next step. Using this syntheticmethodology, the viologen derivatives shown above have been synthesized.

Asymmetric 1-alkyl-1′-alkyl′-4,4′-bipyridinium derivatives can besynthesized in two steps. Generally the reactions are carried out inrefluxing acetonitrile. The first step begins with a slight excess of4,4′-bipyridine (2-5 equivalents) and one equivalent of the alkylfunctionalized with a good leaving group. After the reaction comes tocompletion, the solvent is reduced to a minimal volume, followed byaddition of an aqueous solution of NH₄PF₆. The resulting precipitate iscollected by filtration and washed with water, and then by ether toremove the excess 4,4′-bipyridine. The monofunctionalized bipyridine isthen ready to use in the next step. It is dissolved in refluxingacetonitrile along with the other alkyl group functionalized with a goodleaving group. After the reaction has come to completion, the solvent isreduced to a minimal volume, and an aqueous solution of NH₄PF₆ is added.The resulting precipitate is collected by filtration and washed withwater to yield the product in pure form. Using this syntheticmethodology, the asymmetric viologen derivatives have been synthesized.

Symmetric 1,1′-diaryl-4,4′-bipyridinium derivatives can be synthesizedin two steps. Starting from 4,4′-bipyridinium and two or moreequivalents of 1-chloro-2,4-dinitrobenzene, the mixture is allowed toreflux in acetonitrile for at least three days. After this time, thereaction is allowed to cool to room temperature, and the precipitate iscollected and washed with additional acetonitrile to remove any excess1-chloro-2,4-dinitrobenzene. The resulting chloride salt of1,1′-bis(1,4-dinitrobenzene)-4,4′-bipyridinium is ready to use in thenext step. It is dissolved in a mixture of ethanol and water along withthe desired aryl amine derivative and heated to reflux for a few days.The reaction proceeds by a Zincke mechanism, releasing1,4-dinitroaniline as a side product. After the reaction has come tocompletion, the reaction is filtered in order to remove excess anilinestarting material as well as the side product. The solvent is thenreduced to a minimal level, and an aqueous solution of NH₄PF₆ is added.The resulting precipitate is collected by filtration and washed withwater and ether to afford the target product in pure form. The symmetric1,1′-diaryl-4,4′-bipyridinium shown above have been synthesized in thismanner.

Asymmetric 1-alkyl-1′-aryl-4,4′-bipyridinium derivatives can besynthesized in three steps. Starting with a slight excess of4,4′-bipyridine and one equivalent of 1-chloro-2,4-dinitrobenzene, themixture is allowed to react for a few days in refluxing ethanol. Thesolvent is reduced to a minimal volume, and to it is added an excess ofether. The resulting precipitate is collected by filtration to yield thetarget monofunctionalized bipyridine in pure form. Next, it is dissolvedin an ethanol water mixture along with the desired aryl amine in orderto undergo a Zincke reaction. The solvent is reduced to a minimal volumeand filtered in order to remove excess aniline starting material andside product. To the filtrate is added an aqueous solution of NH₄PF₆,and the resulting precipitate is collected by filtration and washed withwater. The monofunctionalized aryl derivative is ready to use in thenext step. It is dissolved in refluxing acetonitrile along with an alkylderivative functionalized with a good leaving group. After a few days,the solvent is reduced to a minimal level, and an aqueous solution ofNH₄PF₆ is added. The resulting precipitate is collected by filtrationand washed with water and ether, yielding the asymmetric1-alkyl-1′-aryl-4,4′-bipyridinium in pure form. Using this syntheticmethodology, the asymmetric viologen derivative shown above has beensynthesized.

Mechanism of Complex Formation

The chemical formulas and graphical representations of the trisradicaltricationic CBPQT^(2(•+))⊂MV^(•+) complex along with the simplifiedschematic of the redox-induced complexation processes are illustrated inFIG. 1. Following a three-electron reduction, with two electrons, onegoing into each of the two BIPY²⁺ subunits of the CBPQT⁴⁺ ring, and oneelectron going to the BIPY²⁺ unit of the MV²⁺ guest, a stablethermodynamic state arises whereby the CBPQT^(2(•+)) ring is made toencircle the MV^(•+) radical cation. We have shown both experimentallyand theoretically that as a result of the fact that only two of theBIPY^(•+) radical cations of the complex are spin paired at a giventime, the third unpaired BIPY^(•+) radical cation of the complex is notas strongly engaged in radical-radical interactions. This fact hasimportant consequences for the redox-induced mechanism of switching,which will be discussed in detail in this paper. Re-oxidation of thecomplex results in its dissociation into its constituent components.Overall, the complexation is reversible.

In a 1:1 mixture of CBPQT⁴⁺ and MV²⁺, both in their fully oxidizedforms, no binding between the two is observed to occur, as a consequenceof their similar π-electron-poor natures, not to mention electrostaticrepulsion. Upon a three-electron reduction, with two electronstransferred forming the CBPQT^(2(•+)) ring and one forming the MV^(•+)guest, formation of the trisradical CBPQT^(2(•+))⊂MV^(•+) inclusioncomplex ensues promptly as a result of favorable radical-radicalinteractions all of which can be stimulated electrochemically.

The nature of these radical-radical interactions is largely aconsequence of radical pairing, a phenomenon that has been well studiedin the case of the classical viologen radical cation dimers and violenesin general, of which are EPR silent (paired) but are once again activein their monomeric (unpaired) form. This mechanistic feature of radicalpairing has important implications when considering the electrochemicalbehavior of the trisradical CBPQT^(2(•+))⊂MV^(•+) complex. Consider thehypothesis that only two of the BIPY^(•+) radical cation subunits of thecomplex are paired at any one time leaving one BIPY^(•+) radical cationsubunit of the CBPQT^(2(•+)) ring unpaired. As a result, this unpairedBIPY^(•+) unit is not as strongly engaged in radical-radicalinteractions as the other two paired BIPY^(•+) units. Theoreticalinvestigations based on DFT to calculate the singly occupied molecularorbital (SOMO) of the trisradical CBPQT^(2(•+))⊂MV^(•+) complex supportthe hypothesis, by revealing that orbital overlap of the MV^(•+) guestoccurs predominantly with only one of the BIPY^(•+) units in theCBPQT^(2(•+)) ring. The fact that one BIPY^(•+) radical cation in thering is not as strongly interactive with the MV^(•+) guest mandates,from a thermodynamic perspective, that this unpaired BIPY^(•+) unitundergoes reduction to its neutral BIPY⁰ form at a less negativepotential, i.e., easier to reduce—in comparison to the two radicallypaired BIPY^(•+) units. The logical corollary to this is that there-oxidation of the unpaired BIPY^(•+) radical cation unit should occurat more negative potentials i.e., easier to oxidize compared to the twopaired BIPY^(•+) radical cation units. It follows then that theformation of a tetracationic CBPQT^((2+)(•+))⊂MV^(•+) bisradicalinclusion complex must appear in the electrochemical switchingmechanism.

The cyclic voltammogram (CV) of an equimolar mixture of the CBPQT⁴⁺ ringand MV²⁺ is illustrated in FIG. 2 a. The first reduction peak is athree-electron process with two electrons going to CBPQT⁴⁺, forming thediradical dication CBPQT^(2(•+)), and one electron going to MV²⁺,forming the radical cation MV^(•+). As a result of this three-electronprocess, formation of the trisradical tetracationic inclusion complexensues electrochemically. The result of this interaction is that onlyone of the BIPY^(•+) radical cations of the complexed CBPQT^(2(•+))interacts strongly with the MV^(•+) radical cation, such that the secondreduction of this weakly interacting, unpaired BIPY^(•+) occurs at aless negative potential roughly at the same potential as the secondreduction of CBPQT⁴⁺ alone in solution while the second reduction of theBIPY^(•+) of the complexed CBPQT^(2(•+)), paired with the MV^(•+)radical cation, occurs simultaneously at a more negative potential as atwo-electron process.

The [2]rotaxane R⁶⁺ (FIG. 2 b) composed of a CBPQT⁴⁺ ring mechanicallyinterlocked around a dumbbell containing a BIPY²⁺ unit, was obtainedusing a threading-followed-by-stoppering template-directed protocolrelying upon radical-radical interactions and employing a copper-free1,3-dipolar cycloaddition in the final reaction step, which leads to theformation of the mechanical bond. As a consequence of the mechanicalbond, the CBPQT⁴⁺ ring cannot escape the influences of the BIPY²⁺ unitin the dumbbell component entirely. In such a situation, both thetrisradical and bisradical forms of the rotaxane are stable species.Electrochemical experiments reveal that reduction to the trisradicaltricationic form of the rotaxane R^(3(•+)) occurs over two differentelectron-transfer processes. The first is a two-electron reductionprocess, which is assigned to the formation of the spin-paired BIPY^(•+)radical cation of the dumbbell with one from the ring forming thebisradical tetracationic R^(2(•+)(2+)). The subsequent one-electrontransfer is assigned to the reduction of the remaining unpaired BIPY²⁺in the CBPQT^((2+)(•+)) ring, a process which generates the trisradicaltricationic R^(3(•+)) form of the rotaxane. Both of these redoxprocesses are independent of the scan rate, i.e., they are totallyreversible as a consequence of the fact that the mechanical bondexcludes the possibility of dissociation and so serves to stabilize thebisradical form of the rotaxane in comparison to that of its bisradicaltetracationic CBPQT^((2+)(•+))⊂MV^(•+) supramolecular analogue.

In an effort to detect the bisradical tetracationicCBPQT^((2+)(•+))⊂MV^(•+) inclusion complex, variable scan rate CVexperiments were carried out initially on a ⊂MV^(•+) mixture of CBPQT⁴⁺and MV²⁺. They revealed (FIG. 3), however, that the re-oxidation of thetrisradical tricationic complex of CBPQT^(2(•+))⊂MV^(•+) is independent(up to 30 Vs⁻¹) of scan rate. This observation suggests thatdissociation of CBPQT^((2+)(•+))⊂MV^(•+) is too fast to detect on thetimescale of the CV experiments. In order to probe the nature of thebisradical tetracationic intermediate, we performed variable scan rateCV experiments on a 1:1 mixture of CBPQT⁴⁺ and V²⁺, where the butynylfunctions act to slow down the rate of dissociation such that theintermediate can be more readily observed under the experimental setupemployed. We hypothesize that the extra steric bulk and/or favorablenoncovalent bonding interactions resulting from the presence of thebutynyl substituents of V²⁺ act to stabilize the bisradicaltetracationic intermediate kinetically CBPQT^((2+)(•+))⊂V^(•+) incomparison to the methyl groups of MV²⁺.

Evidence for the existence of CBPQT^((2+)(•+))⊂V^(•+) inclusion complexin solution can be obtained (FIG. 4) from variable scanrate CV in MeCN.As a consequence of the presence of the dicationic BIPY²⁺ unit, thebisradical CBPQT^((2+)(•+))⊂V^(•+) complex forfeits a substantial amountof stability compared to the trisradical CBPQT^(2(•+))⊂V^(•+) complex.The CBPQT^((2+)(•+))⊂V^(•+) begins to dissociate into is individualcomponents, making re-oxidation of the two remaining BIPY^(•+) radicalcations of the separate host and guest components occur at a morenegative (easier to re-oxidize) potential. When scanning at a rateslower than the timescale of the dissociation of theCBPQT^((2+)(•+))⊂V^(•+), re-oxidation of the trisradical tricationiccomplex is observed to occur as a single, broad oxidation wave. Whenperforming the re-oxidation at progressively faster scanrates—eventually reaching a point where dissociation of theCBPQT^((2+)(•+))⊂V^(•+) is slow on the CV timescale—re-oxidation of thetrisradical tricationic complex is observed to occur over (i) aone-electron transfer, followed (ii) by another two-electrontransfer—the latter of which occurs at a potential shifted to almost 0V! The first one-electron oxidation, leading to the formation ofCBPQT^((2+)(•+))⊂V^(•+) is assignable to the unpaired BIPY^(•+) subunitof the trisradical trication complex, which is more weakly engaged inradical-radical interactions. The second, dramatically shiftedtwo-electron process, can be assigned to the simultaneous two-electronoxidation of the radically paired BIPY^(•+) units ofCBPQT^((2+)(•+))⊂V^(•+), leading to the fully oxidized and highlyunstable hexacationic CBPQT⁴⁺⊂V²⁺ complex, which quickly dissociates onthe timescale of the CV experiment.

As a consequence of the disassociation of the CBPQT^((2+)(•+))⊂V^(•+)inclusion complex, we hypothesize that the re-oxidation potentials forboth separated components become shifted to more negative potentialsresembling those of the free components alone in solution than the firstone-electron oxidation of the unpaired BIPY^(•+) radical cation in thescan, resulting in a single broad re-oxidation peak. Mechanistically, wepropose a first-order rate law for the dissociation of theCBPQT^((2+)(•+))⊂V^(•+) inclusion complex into its individual componentsthat is proportional to a rate constant and the concentration of thebisradical species. As a control, the rotaxane R⁶⁺ containing theCBPQT⁴⁺ ring and a dumbbell incorporating only a BIPY²⁺ unit was alsoinvestigated (FIG. 5) by variable scan rate CV. The [2]rotaxane R⁶⁺shows no variable scan rate dependent behavior.

Digital CV simulations of the proposed mechanism were performed (FIG.4), based on the results obtained from ITC and stopped-flowspectroscopy, and are in good agreement with the experimental data.Comparisons against digital simulations to the experimental data using aX² fitting algorithm, or by employing a relative integration analysis(FIG. 6) of the re-oxidation waves as a function of time (scanrate) thebarrier governing the first-order dissociation of the bisradicaldicationic complex is established to be 16.0 kcal mol⁻¹ or 16.5 kcalmol⁻¹, respectively at room temperature. Table 1 summarizes the kineticparameters obtained from stopped-flow UV/Vis spectroscopic and CV data.It is worthy to note, that since the kinetic re-oxidation pathway of thetrisradical tricationic complex is scan rate dependent, it represents anexample of a bilabile system.

BIPY Guest Compound

The complexes described herein are guest-host type complexes, andcomprise a CBPQT^(2(•+)) and a BIPY^(•+) compound of formula (I):

wherein the complex has a 3⁺ charge and three radicals, andR¹ and R² are each independently selected from alkyl, cycloalkyl,heterocycloalkyl, aryl, heteroaryl, alkenyl, alkynyl, alkyleneazido,alkylenecycloalkyl, alkyleneheterocycloalkyl, or alkylenearyl, or a saltthereof. The BIPY can have a +charge and radical, and the CBPQT a2+charge and two radicals.

Selection of the R¹ and R² allow for tunability of the complexproperties, for example, by modifying the electron withdrawing anddonating characteristics of the R groups. The choice of R groups willinfluence the resulting conductance, redox potential, and UV-vischaracteristics of the complexes.

Some specific compounds contemplated include, but are not limited to

(shown before reduction to the cation radical form). Some specific Rgroups for the compound of formula (I) include

These are but only a few representative examples of the possible alkylgroups. Other examples include where the above functional groups havebeen modified by halogens in some position(s), with hydroxyl groups,with carboxylates, with esters, with amines, with amides, with azides,and with ethers. The limitation to the alkyl-based substitution occurswhen the alkyl chains becomes so long that the large degree of freedomassociated with rotation about the sp³ C—C bonds prevents efficientcrystallization from occurring.

Possible aryl substituents to the 4,4′ positions of bipyridine are shownbelow and include phenyl, naphthyl, and pyrenyl derivatives. Upon thesearyl substituents, additional functional groups of eitherelectron-donating or electron-withdrawing natures may be covalentlyattached. By varying the electron-donating or electron withdrawingfunctional groups, the electronic and UV/spectroscopic properties of thesubstrate can be tuned to fit the needs of the given application. Thesefunctional groups are shown below, and more than one or differentcombinations of them may be imposed on the aryl substituents. Thelimitation to this functionalization occurs when the resultingdimensions of the functionalized aryl-substituent becomes sterically toolarge such that threading of the CBPQT^(2(•+)) ring can no longer occur.

In consideration that the approximate dimensions of the CBPQT^(2(•+))ring are approximately 10 Å×7 Å, the following functionalized arylsubstituents represent the limit of steric bulk imposed such thatanything sterically larger will prevent the CBPQT^(2(•+)) fromthreading.

The following are examples of functionalized aryl substituents which areabove the steric limit for threading of the CBPQT^(2(•+)) from to occur.

In these specific cases, a clipping mechanism must be pursued ratherthan a threading mechanism in order to see the CBPQT^(2(•+)) encircledaround the central BIPY^(•+) core.

The term “alkyl” used herein refers to a saturated or unsaturatedstraight or branched chain hydrocarbon group of one to forty carbonatoms, including, but not limited to, methyl, ethyl, n-propyl,isopropyl, n-butyl, isobutyl, tert-butyl, n-hexyl, and the like. Alkylsof one to six carbon atoms are also contemplated. The term “alkyl”includes “bridged alkyl,” i.e., a bicyclic or polycyclic hydrocarbongroup, for example, norbornyl, adamantyl, bicyclo[2.2.2]octyl,bicyclo[2.2.1]heptyl, bicyclo[3.2.1]octyl, or decahydronaphthyl. Alkylgroups optionally can be substituted, for example, with hydroxy (OH),halide, thiol (SH), aryl, heteroaryl, cycloalkyl, heterocycloalkyl, andamino. It is specifically contemplated that in the compounds describedherein the alkyl group consists of 1-40 carbon atoms, preferably 1-25carbon atoms, preferably 1-15 carbon atoms, preferably 1-12 carbonatoms, preferably 1-10 carbon atoms, preferably 1-8 carbon atoms, andpreferably 1-6 carbon atoms. “Heteroalkyl” is defined similarly asalkyl, except the heteroalkyl contains at least one heteroatomindependently selected from the group consisting of oxygen, nitrogen,and sulfur.

As used herein, the term “cycloalkyl” refers to a cyclic hydrocarbongroup, e.g., cyclopropyl, cyclobutyl, cyclohexyl, and cyclopentyl.“Heterocycloalkyl” is defined similarly as cycloalkyl, except the ringcontains one to three heteroatoms independently selected from the groupconsisting of oxygen, nitrogen, and sulfur. Nonlimiting examples ofheterocycloalkyl groups include piperdine, tetrahydrofuran,tetrahydropyran, dihydrofuran, morpholine, thiophene, and the like.Cycloalkyl and heterocycloalkyl groups can be saturated or partiallyunsaturated ring systems optionally substituted with, for example, oneto three groups, independently selected from the group consisting ofalkyl, alkyleneOH, C(O)NH₂, NH₂, oxo (═O), aryl, haloalkyl, halo, andOH. Heterocycloalkyl groups optionally can be further N-substituted withalkyl, hydroxyalkyl, alkylenearyl, or alkyleneheteroaryl.

The term “alkenyl” used herein refers to a straight or branched chainhydrocarbon group of two to ten carbon atoms containing at least onecarbon double bond including, but not limited to, 1-propenyl,2-propenyl, 2-methyl-1-propenyl, 1-butenyl, 2-butenyl, and the like. Theterm “cycloalkenyl” refers to a cycloalkyl group having one or moredouble bonds. “Heterocycloalkenyl” refers to a cycloalkenyl group havingone or more heteroatoms (e.g., N, S, O, or combinations thereof).

The term “alkynyl” used herein refers to a straight or branched chainhydrocarbon group of two to ten carbon atoms containing at least onecarbon triple bond including, but not limited to, 1-propynyl,2-propynyl, 1-butynyl, 2-butynyl, and the like.

The term “halide” or “halo” used herein refers to fluoro, chloro, bromo,or iodo.

The term “alkylene” used herein refers to an alkyl group having asubstituent. For example, the term “alkylene aryl” refers to an alkylgroup substituted with an aryl group. The alkylene group is optionallysubstituted with one or more substituent previously listed as anoptional alkyl substituent. For example, an alkylene group can be—CH₂CH₂— or CH₂—.

As used herein, the term “aryl” refers to a monocyclic or polycyclicaromatic group, preferably a monocyclic or bicyclic aromatic group,e.g., phenyl or naphthyl. Unless otherwise indicated, an aryl group canbe unsubstituted or substituted with one or more, and in particular oneto four groups independently selected from, for example, halo, alkyl,alkenyl, OCF₃, NO₂, CN, NC, OH, alkoxy, amino, CO₂H, CO₂alkyl, aryl, andheteroaryl. Exemplary aryl groups include, but are not limited to,phenyl, naphthyl, tetrahydronaphthyl, chlorophenyl, methylphenyl,methoxyphenyl, trifluoromethylphenyl, nitrophenyl,2,4-methoxychlorophenyl, and the like.

As used herein, the term “heteroaryl” refers to a monocyclic or bicyclicring system containing one or two aromatic rings and containing at leastone nitrogen, oxygen, or sulfur atom in an aromatic ring. Unlessotherwise indicated, a heteroaryl group can be unsubstituted orsubstituted with one or more, and in particular one to four,substituents selected from, for example, halo, alkyl, alkenyl, OCF₃,NO₂, CN, NC, OH, alkoxy, amino, CO₂H, CO₂alkyl, aryl, and heteroaryl. Insome cases, the heteroaryl group is substituted with one or more ofalkyl and alkoxy groups. Examples of heteroaryl groups include, but arenot limited to, thienyl, furyl, pyridyl, oxazolyl, quinolyl, thiophenyl,isoquinolyl, indolyl, triazinyl, triazolyl, isothiazolyl, isoxazolyl,imidazolyl, benzothiazolyl, pyrazinyl, pyrimidinyl, thiazolyl, andthiadiazolyl.

The term “alkoxy” used herein refers to straight or branched chain alkylgroup covalently bonded to the parent molecule through an —O— linkage.Examples of alkoxy groups include, but are not limited to, methoxy,ethoxy, propoxy, isopropoxy, butoxy, n-butoxy, sec-butoxy, t-butoxy andthe like.

The term “thioalkyl” used herein refers to one or more thio groupsappended to an alkyl group.

The term “thioether” used herein refers to straight or branched chainalkyl or cycloalkyl group covalently bonded to the parent moleculethrough an —S— linkage. Examples of thioether groups include, but arenot limited to, —SCH₃, —SCH₂CH₃, —SCH₂CH₂CH₃, —SCH(CH₃)₂,—SCH₂CH₂CH₂CH₃, —SCH₂CH(CH₃)₂, —SC(CH₃)₃ and the like.

The term “hydroxyalkyl” used herein refers to one or more hydroxy groupsappended to an alkyl group.

The term “azide” refers to a —N₃ group. The term “nitro” refers to a—NO₂ group.

The term “amino” as used herein refers to —NR₂, where R is independentlyhydrogen, optionally substituted alkyl, optionally substitutedheteroalkyl, optionally substituted cycloalkyl, optionally substitutedheterocycloalkyl, optionally substituted aryl or optionally substitutedheteroaryl. Non-limiting examples of amino groups include NH₂, NH(CH₃),and N(CH₃)₂. In some cases, R is independently hydrogen or alkyl.

The term “amido” as used herein refers to —C(O)NH₂, —C(O)NR₂, —NRC(O)Ror —NHC(O)H, where each R is independently hydrogen, optionallysubstituted alkyl, optionally substituted heteroalkyl, optionallysubstituted cycloalkyl, optionally substituted heterocycloalkyl,optionally substituted aryl or optionally substituted heteroaryl. Insome cases, the amido group is —NHC(O)alkyl or —NHC(O)H. In variouscases, the amido group is —C(O)NH(alkyl) or —C(O)NH(substituted alkyl).A non-limiting example of an amido group is —NHC(O)CH₃.

As used herein, a substituted group is derived from the unsubstitutedparent structure in which there has been an exchange of one or morehydrogen atoms for another atom or group. A “substituent group,” as usedherein, means a group selected from the following moieties:

(A) —OH, —NH₂, —SH, —CN, —CF₃, —NO₂, oxo, halogen, unsubstituted alkyl,unsubstituted heteroalkyl, unsubstituted cycloalkyl, unsubstitutedheterocycloalkyl, unsubstituted aryl, unsubstituted heteroaryl,unsubstituted alkoxy, unsubstituted aryloxy, trihalomethanesulfonyl,trifluoromethyl, and

(B) alkyl, heteroalkyl, cycloalkyl, heterocycloalkyl, aryl, amino,amido, carbonyl, thiocarbonyl, alkoxycarbonyl, silyl, sulfonyl,sulfoxyl, alkoxy, aryloxy, and heteroaryl, substituted with at least onesubstituent selected from:

-   -   (i) —OH, —NH₂, —SH, —CN, —CF₃, —NO₂, oxo, halogen, unsubstituted        alkyl, unsubstituted heteroalkyl, unsubstituted cycloalkyl,        unsubstituted heterocycloalkyl, unsubstituted aryl,        unsubstituted heteroaryl, unsubstituted alkoxy, unsubstituted        aryloxy, trihalomethanesulfonyl, trifluoromethyl, and    -   (ii) alkyl, heteroalkyl, cycloalkyl, heterocycloalkyl, aryl,        amino, amido, carbonyl, thiocarbonyl, alkoxycarbonyl, silyl,        sulfonyl, sulfoxyl, alkoxy, aryloxy, and heteroaryl, substituted        with at least one substituent selected from:        -   (a) —OH, —NH₂, —SH, —CN, —CF₃, —NO₂, oxo, halogen,            unsubstituted alkyl, unsubstituted heteroalkyl,            unsubstituted cycloalkyl, unsubstituted heterocycloalkyl,            unsubstituted aryl, unsubstituted heteroaryl, unsubstituted            alkoxy, unsubstituted aryloxy, trihalomethanesulfonyl,            trifluoromethyl, and        -   (b) alkyl, heteroalkyl, cycloalkyl, heterocycloalkyl, aryl,            amino, amido, carbonyl, thiocarbonyl, alkoxycarbonyl, silyl,            sulfonyl, sulfoxyl, alkoxy, aryloxy, and heteroaryl,            substituted with at least one substituent selected from —OH,            —NH₂, —SH, —CN, —CF₃, —NO₂, oxo, halogen, unsubstituted            alkyl, unsubstituted heteroalkyl, unsubstituted cycloalkyl,            unsubstituted heterocycloalkyl, unsubstituted aryl,            unsubstituted heteroaryl, unsubstituted alkoxy,            unsubstituted aryloxy, trihalomethanesulfonyl,            trifluoromethyl.

The term “carboxy” or “carboxyl” used herein refers to —COOH or itsdeprotonated form —COO⁻. C₁₋₁₀carboxy refers to optionally substitutedalkyl or alkenyl groups having a carboxy moiety. Examples include, butare not limited to, —CH₂COOH, —CH₂CH(COOH)CH₃, and —CH₂CH₂CH₂COOH.

In some cases, the substituent group(s) is (are) one or more group(s)individually and independently selected from alkyl, cycloalkyl, aryl,heterocyclyl, heteroaryl, hydroxy, alkoxy, aryloxy, mercapto, alkylthio,arylthio, cyano, halo, carbonyl, thiocarbonyl, alkoxycarbonyl, nitro,silyl, trihalomethanesulfonyl, trifluoromethyl, and amino, includingmono and di substituted amino groups, and the protected derivativesthereof.

The protecting groups that can form the protective derivatives of theabove substituents are known to those of skill in the art and can befound in references such as Greene and Wuts, Protective Groups inOrganic Synthesis; 3^(rd) Edition, John Wiley and Sons: New York, 2006.Wherever a substituent is described as “optionally substituted” thatsubstituent can be substituted with the above-described substituents.

Mechanism of Switching in MIMs

The reduction portion of the CV for the [2]rotaxane R1⁶⁺ displayssimilar behavior to the that of the trisradical inclusion complex.Following a three-electron reduction, the diradical dicationicCBPQT^(2(•+)) ring is induced to translate from the 1,5-dioxynaphthalene(DNP) recognition site to encircle the radical cation BIPY^(+•)recognition unit forming the trisradical state as has been previouslydemonstrated. The re-oxidation of R1⁶⁺, like the trisradicalCBPQT^(2(•+))⊂V^(•+) complex is again scan rate dependent. Similarly, aone-electron oxidation process leading to the formation of thebisradical is observed followed by a two-electron re-oxidation, whichcan be observed at relatively fast scan rates corresponding to theoxidation of the radically paired bisradical to the fully oxidizedstate, which likewise immediately leads to the formation of the groundstate, where the CBPQT⁴⁺ ring is encircling the DNP unit. However, inthe case of the [2]rotaxane R1⁶⁺, we observed a difference in thedependence of the second two-electron re-oxidation peak upon changingthe scan rate, namely that the presence of the second oxidation peakcould be easily observed even at scan rates as slow as 10 mV s⁻¹ (FIG.7). We propose a mechanism that after the one-electron oxidation of theweakly interacting unpaired BIPY^(+•) of the CBPQT^(2(•+)) ringencircling the BIPY^(•+) radical cation of the dumbbell component, thisnow dicationic BIPY²⁺ of the CBPQT^((•+)(2+)) ring is now able torecognize the DNP unit through donor-acceptor interactions. Like thepseudorotaxane, the one-electron oxidation causes the formation of ametastable intermediate, the corresponding bisradical, which in the caseof the [2]rotaxane R1⁶⁺, leads to a mechanical movement of the ring inorder that it may encircle the DNP as the corresponding R-ground stateas consequence of it being the most thermodynamically favorable positiondue to a combination of favorable donor-acceptor interactions andunfavorable columbic repulsion.

In order to support this mechanism of relaxation from the radicallypaired bisradical to the unpaired bisradical, we have also synthesizedand studied by variable scan rate CV the reduction behavior of a singlestation[2]rotaxane incorporating only a BIPY²⁺ moiety in the dumbbellcomponent with no other accompanying pi-donor recognition stations. Inthe case of this [2]rotaxane, we expect that no relaxation would occur,which would result in observing no scan rate dependent behavior in thereduction region of the CV. Indeed, this is the case. The relativeintensities of the first and second re-oxidation peaks were scan rateindependent (see SI for further discussion).

Although in the case of the [2]rotaxane R1⁶⁺ “one-half” of theCBPQT^((•+)(2+)) ring is still able to recognize the BIPY^(•+) radicalcation of the dumbbell component, we hypothesize that the presence ofthe dication in the CBPQT^((•+)(2+)) ring acts to further lower thestability of the radically paired bisradical translational isomer makingthe population distribution of the radically spin-paired bisradicalessentially zero at thermodynamic equilibrium. Following translation ofthe CBPQT^((•+)(2+)) to the DNP moiety, the re-oxidation potentials ofthe remaining BIPY^(•+) radical cations of the R-GSCC shift to morenegative potentials, and so the second two-electron re-oxidation peak isobserved to decrease at slower scan rates, just as was the case for the[2]pseudorotaxane CBPQT^(2(•+))⊂V^(•+). Quantifying the amount ofradically paired bisradical intermediate as a function of time, we cangenerate the decay profile shown in FIG. 7 b. We propose a first-orderrelaxation mechanism, in which the rate of relaxation is dependent onlyon a rate constant and the concentration of the radically pairedbisradical. Fitting the data to a first-order decay profile and usingthe extracted time constant in the Eyring equation reveals the freeenergy barrier to relaxation ΔG^(‡) to be 19.1 kcal mol⁻¹. From thesekinetic data we can construct a thermodynamic profile shown if FIG. 7 cgoverning the relaxation of the radically paired to the unpairedbisradical that occurs after the initial one-electron oxidation.

Use of Reducing Agents to Form Complexes

Zinc dust was activated by stirring with dilute HCl during 10-15minutes, and then washed several times with distilled water, ethanol,and absolute diethyl ether before rigorous drying. This procedureremoves oxides from the surface of zinc, which form slowly upon standingin air. All the studies were carried out with spectroscopic gradeacetonitrile (Acros Organics ≧99.9% for spectroscopy). All the stocksolutions of CPBQT.4PF₆ and MV.2PF₆ were prepared by weighing using anAG 245 Mettler Toledo analytical balance (precision 0.01 mg) andcomplete dissolution in MeCN was achieved using an ultrasonic bath(Bandelin Sonorex RK102 Transistor). Their concentrations were thusobtained by weighing the appropriate amounts. Reduction of CPBQT⁴⁺ andMV²⁺ into the corresponding bisradical CPBQT^(2+•) and monoradicalMV^(+•) was achieved under argon (CO₂-free and O₂-free argon using aSigma Oxiclear cartridge) in less than one hour by vigorous stirringwith activated zinc dust and was monitored using absorptionspectrophotometry. After which time, the zinc dust is filtered of usinga 0.45 μm syringe filter still inside of the glovebox. The coloredsolution is then ready to use without further purification.

Other reducing agents can be used, more particularly reducing agentswith an oxidation potential that is compatible with the reduction ofCPBQT⁴⁺ and the compound of formula (I) to the tris-radical di-cationcomplex. In the electrochemical series, metals at the top of the seriesare good at giving away electrons. They are good reducing agents. Thereducing ability of the metal increases as one increases across theseries. Therefore, all the metals which are placed above Zinc in theelectrochemical series or in other words, metals which posses oxidationpotential higher than Zn (0.76 V) can be used as the reductant. Forexample Li (3.04 V), K (2.92 V), Ba (2.90 V), Ca (2.76 V), Na (2.71 V),Mg (2.37 V), A1 (1.66 V), Mn (1.18 V) can be used as a reductant. Thevalues of standard electrode potentials are given in volts relative tothe standard hydrogen electrode. It is important to note, however, thatthese reducing agents are likely strong enough to reduce the CBPQT⁴⁺ring and BIPY²⁺ threads to their neutral states, CBPQT⁰ and BIPY⁰,respectively. In order to arrive at the radical cation state, the neutalCBPQT⁰ and BIPY⁰ following reduction are combined with a stoichiometricequivalent of CBPQT⁴⁺ and BIPY²⁺. At this point the neutal CBPQT⁰ andBIPY⁰ will undergo electronic dispropotionation with CBPQT⁴⁺ and BIPY²⁺resulting in a solution containing only the CBPQT^(2(•+)) and BIPY^(•+)radical cations.

The following is a list of other common reducing agents that could bepotentially used to reduce CBPQT⁴⁺ ring and BIPY²⁺ threads to theirneutral states: Nacent hydrogen, sodium amalgam, NaBH₄, sulfite salts(e.g. Na₂S₂O₄), Zn(Hg) alamgam, oxalic acid, formic acid, ascorbic acid,and coboltocene.

Reducing agents that are less preferred, due to possible side-reactionsunder many commonly employed conditions, are lithium aluminium hydride(LAH), SnCl₂, hydrazine, and, diisobutylaluminium hydride (DIBAL-H).

Crystallization Techniques to Form Crystals of Complexes

Single crystals of the CBPQT^(2(•+))⊂MV^(•+) complex were grown fromMeCN using slow-vapor diffusion of iPr₂O under inert conditions, i.e.,inside a glovebox, at room temperature. We used zinc dust to affect thereduction of the CBPQT⁴⁺ and MV²⁺ to their respective radical cationforms. The solid-state structure as determined by X-ray crystallographyfurther supports (FIG. 8) the formation of the trisradical tricationiccomplex. The inclusion complex was observed to be associated with threehexflurorophosphate counteranions in the solid-state, supporting thehypothesis that each of the BIPY²⁺ units is reduced to its radicalcation form. The MV^(•+) is situated inside the cavity of theCBPQT^(2(•+)) ring in a centrosymmetrtic fashion, with 3.22 Åcentroid-to-centroid transannular separation from each of the BIPY^(•+)radical cation subunits of the cyclophane, with an 14° angle of offset(FIG. 8) from the plane orthogonal to the one defined by the four Natoms of the ring. We hypothesize this deviation from orthoganilitymaximizes the amount of π-overlap between MV^(•+) and CBPQT^(2(•+)). Notorsional twisting of any of the three BIPY^(•+) units of the complexabout their 4,4′-C—C bond is apparent, a phenomenon which is notgenerally observed for complexes involving the CBPQT⁴⁺ ring at least inits fully oxidized form when bound with electron-rich guests. Thesestructural details are in good agreement with the structure predictedfrom theoretical calculations reported on previously. The extendedsuperstructure reveals a continuous BIPY^(•+) radical cation π-stack,with adjacent complexes lying in register “shoulder-to-shoulder” 3.28 Åapart from each other.

As a control, we investigated the crystal growth of the CBPQT^(2(•+))alone in solution. Crystals were readily obtained from slow-vapordiffusion of iPr₂O in a solution of CBPQT^(2(•+)) in MeCN. Remarkably,long dark, opaque needle-like crystals up to an inch in length wereobserved after less than a week of crystal growth. Single crystal X-raycrystallographic analyses revealed (FIG. 9) first of all that the ringcrystallized from solution into the solid-state as the diradicaldication CBPQT^(2(•+)) as can be reckoned from the PF₆ ⁻ counteranioncount. No torsional twist was again observed in the BIPY^(•+) units ofthe CBPQT^(2(•+)) ring, an observation which is consistent with thatobserved for the trisradical complex. The centroid-to-centroidtransannular distance spanning the length between the two BIPY^(•+)units of the ring is 6.92 Å, a remarkably longer distance compared tothe trisradical complex, whose analogous distance is 6.43 Å. Theseobservations indicate that the CBPQT^(2(•+)) ring is able to flex inorder to accommodate the MV^(•+) guest in such a way that reduces thebowing of the BIPY^(•+) units of the ring. As a further consequence tothis, the centroid-to-centroid transannular separation between thephenylene units observed in the X-ray structure of the CBPQT^(2(•+))ring is 10.21 Å, while the analogous distance for the trisradicalcomplex is 10.34 Å. The decreased degree of bowing of the BIPY^(•+)units brought about by inclusion of MV^(•+) serves to increase thedistance between the phenylene units of the ring. The superstructure,much like the way the trisradical complex packs in the solid-state,reveals a continuous stack of CBPQT^(2(•+)) rings spaced 3.12 Å apartarranged shoulder-to-shoulder with a zero degree angle of offsetalthough with void spaces arising from the unoccupied cavities of therings! In fact, the cavities of the rings arrange in such a way as toform continuous porous channels that run the length of the crystal, withthe PF₆ ⁻ counterions occupying the space between the rings. Overall,the superstructure is consistent with our understanding of the nature ofBIPY^(•+) radical-radical interactions, which serve to “stitch” theCBPQT^(2(•+)) rings together into continuous stacks.

It is possible by varying the substitution of the viologen radicalcation thread in the 4,4′ positions, using either alkyl-based oraryl-based functionalities, the electronic, UV/Vis spectroscopic andsteric properties of the resulting trisradical complexes can bemodified, and hence the solid-state properties can be tuned.

Crystallization of Surfaces and Analysis with Scanning ElectronMicroscopy

Crystallization of the trisradical trication inclusion complexCBPQT^(2(•+))⊂MV⁺ can be made to occur on silicon surfaces usingreduction with Zn dust and slow-vapour diffusion methodologiespreviously described. Analysis with Scanning Electron Microscopy (SEM)reveals that these crystals form long molecular wires severalmicrometers long. The length of the crystal wires can be controlled byvarying the concentration of the solution of CBPQT^(2(•+))⊂MV⁺ prior toslow-vapor diffusion. In particular, larger concentrations lead tolarger crystals. Visual inspection of crystals grown large enough to beseen by the naked eye reveals that the crystals are black, that is, theystrongly absorb wavelengths across the visible spectrum. These crystalshave been incorporated into a field effect transistor (FET) device (FIG.15) in order to determine their conductivity. These experiments revealthat the crystals are conductive (FIG. 16) and act as p-typesemiconductors. The fact that these crystals are conductive and stronglyabsorb light in the visible region renders them with considerablepotential to serve as the active organic component in eithersingle-layer organic solar cells (SLOSCs). SLOSCs have fallen out offavor as a consequence of their exactions tendency to recombine beforethey are able to be collected at their respective electrodes. This islikely a result of the low long-range order of the organic materialbeing employed, usually a conjugated polymer. We propose to construct(FIG. 17) SLOSCs with these crystals by the following methodology. Firstof all, we have shown by EPR that the trisradical CBPQT^(2(•+))c⊂MV^(•+)complex is paramagnetic in solution. Therefore, application of amagnetic field (B) during crystal growth is likely to result in growthof the crystal along a direction a vector dictated by the direction ofthe applied magnetic field. In this way, we expect to be able to grow alayer of crystals all aligned in a similar direction. Performing thisprocedure on an aluminum surface acting as the bottom electrode isexpected to result in an ordered array of crystals (FIG. 17 a).Following this, a layer of indium tin oxide (ITO) and glass on top ofthe crystals will serve as the other electrode. Upon irradiation withlight (FIG. 17 b), a photocurrent is expected to result with electronsflowing to the ITO layer and holes propagating to the aluminum layer.Since SLOSC are intrinsically simpler by design than the currentlyfavored double-layer donor-acceptor organic solar cells, the designproposed herein will offer a considerable advantage. Examples of crystalnanowires are shown in FIG. 18.

Characterization by Isothermal Titration calorimetry and UV/VisSpectroscopy

We were able to obtain thermodynamic parameters governing the stabilityof the trisradical complex by performing ITC experiments at roomtemperature in MeCN under the inert conditions provided by a glove box.Solutions of CBPQT^(2(•+)) and MV^(•+) were prepared individually usingzinc dust as the reducing agent. Results (FIG. 10) from ITC confirm the1:1 stoichiometry of the complex and reveal an association constant(K_(a)) value of 5.04±0.63×10⁴ M⁻¹. As expected, the binding isenthalpically driven with a ΔH value of −15.2±0.5 kcal mol⁻¹ with anentropic cost of −29.3±1.6 cal mol⁻¹ K⁻¹ for ΔS.

The absorption spectra of the radical cation MV^(+•) and of thediradical dication CPBQT^(2(•+)) were first of all recorded alone insolution at concentrations of ˜5×10⁻⁵-10⁻⁴ M to verify the absence ofintermolecular radical-radical interactions between like species underthe experimental conditions employed. The absorption spectra of MV^(+•)and of CPBQT^(2(•+)) are both characterized (FIG. 11) by two sets offinely-structured absorptions (vibronic coupling) centered on ˜390 nmand 600 nm, respectively. No absorption was observed in the near IRregion, an observation which substantiates the absence of intermolecularradical-radical interactions—namely, dimerization—in MeCN. Theelectronic spectrum of CPBQT^(2(•+)) was compared to the sum of theelectronic spectra of two stoichiometric equivalents of MV^(+•) in orderto investigate the ability of the two BIPY^(•+) units in CPBQT^(2(•+))to interact in a noncovalent fashion. The larger extinction coefficientsof the CPBQT^(2(•+)) diradical dication compared to the sum of twoMV^(+•) radical cations, suggests an intramolecular dipole-dipoleinteraction in the ground state between the CPBQT^(2(•+)) ring's twoBIPY^(+•) radical cation units, which are held rigidly apart at adistance of approximately 7 Å within the CPBQT^(2(•+)) diradicaldiaction. This observation is in good agreement with EPR resultspreviously obtained, data of which supports the hypothesis ofintramolecular dipole-dipole interactions.

The strength of the association between the CPBQT^(2(•+)) and theMV^(•+) was probed in MeCN using UV/Vis absorption spectrophotometrywhich allowed us to assess the spectroscopic and thermodynamicparameters associated with the formation of the CPBQT^(2(•+))⊂MV^(•+)trisradical trication. The recognition of MV^(•+) by CPBQT^(2(•+))occurs with a significant broadening of the absorption band centered atca. 604 nm, resulting effectively in the observation of a new bandappearing (FIG. 12) around 550 nm, and in the formation of an intenseabsorption band straddling 1075 nm. The processing of thesespectrophotometric data allowed for a further determination of thestability constant (K_(a)) for the trisradical trication inclusioncomplex, in addition to affording the corresponding electronic spectrum.The electronic spectrum of the 1:1 trisradical tricationicCPBQT^(2(•+))⊂MV^(•+) complex displays the characteristic spectroscopicfeatures of BIPY^(•+) radical-radical interactions (pimerization)—namelya broad NIR band as well as an increase in intensity in the 500-600 nmregion when referenced to the free components alone in solution. Theresults from this titration reveal that the K_(a) value is 7.9±5.5×10⁴M⁻¹, an association constant which is consistent within experimentalerror of the binding constant determined from ITC experiments. Table 1summarizes the thermodynamic parameters obtained from the ITC and UV/Visspectroscopic data.

Mechanism of Mechanical Bond Formation byThreading-Followed-by-Stoppering

The successful template-directed synthetic strategy for the assembly ofthe [2]rotaxane R.6PF₆ is illustrated in FIG. 13. The viologenderivative T²⁺ with two terminal azide functions was prepared in highoverall yield in three steps from 11-bromo-1-undecanol and4,4′-bipyridine. The template-directed synthesis of the [2]rotaxaneR.6PF₆ was achieved using a copper free azide-alkyne 1,3-dipolarcycloaddition after the BIPY²⁺ dications in both CBPQT⁴⁺ and T²⁺ hadbeen reduced to BIPY^(•+) radical cations to promote the formation ofthe T^(•+)⊂CBPQT^(2(•+)) inclusion complex. In order to effect thereduction of all three BIPY²⁺ units simultaneously to their respectiveradical cations in both T^(•+) and CBPQT^(2(•+)), the well known[Ru(bpy)₃]²⁺ reducing system which can be activated by visible light,was chosen because of its highly efficient reduction of BIPY²⁺ unitsusing photo-induced charge-transfer. Triethanolamine, which was employedas the sacrificial electron donor, prevents back electron transfer fromthe BIPY^(•+) radical cation to the [Ru(bpy)₃]³⁺ species. In order tocomplete the template-directed synthesis of the [2]rotaxane by athreading-followed-by-stoppering strategy, a 1,3-dipolar cycloadditionbetween di-tert-butyl acetylenedicarboxylate the precursor to the twostoppers and the terminal azide groups on the thread component of theinclusion complex was performed. The [2]rotaxane R.6PF₆ was isolated in35% yield after a work-up during which time all the BIPY^(•+) radicalcations were oxidized back to BIPY²⁺ dications by atmospheric oxygen.

Mechanism of Mechanical Bond Formation by Clipping

The successful template-directed strategy employing a clipping mechanismin order to synthesize the homo[2]catenane C⁴⁺ is shown in FIG. 14. Thesynthesis begins with a 1:1 mixture of the CBPQT⁴⁺ ring and the benzylbromide viologen derivative VB²⁺ in degassed MeCN under an inertatmosphere (glove box). To this solution is added an excess of Zn dustin order to affect the reduction of the two components to their radicalcation forms and induce their complexation. The Zn dust is then filteredoff, and to the solution is added an equivalent of 4,4′-bipyridine. Wepropose the reaction proceeds through an oligomeric intermediate priorto cyclization forming the final interlocked product. Afterpurification, the homo[2]catenane is isolated in 20% yield.

EXAMPLES Example 1

MV.2PF₆: To a solution of 4,4′-bipyridine (1 g, 6.4 mmol) in MeCN (50mL) was added MeI (2.7 g, 19.2 mmol). The reaction was allowed to stirfor two days under reflux, during which time a precipitate was formed.The solvent was reduced to a minimal volume, and to it was added asolution (50 mL) of aqueous NH₄PF₆. The resulting precipitate wascollected by filtration and washed with H₂O and ether. The resultingyellow solid was found to be the target product in pure form. Collected3.0 g (98%).

Example 2

NPV.2PF₆: To a solution of 4,4′-bipyridine (1 g, 6.4 mmol) in MeCN (50mL) was added 1-chloro-2,4-dinitrobenzene (3.9 g, 19.2 mmol). Thereaction was set to reflux for 3 d, during which time a precipitate wasobserved to form. At the end of 3 d, the reaction was allowed to cool toroom temperature, and the precipitate was collected by filtration andwashed with additional MeCN (25 mL). The solid was allowed to dry undervacuum before being dissolved in a 30:70 mixture of EtOH/H₂O alsodissolved with 1-amino-5-hydroxynaphthalene (3.1 g, 19.2 mmol). Thereaction mixture was set to reflux for 5 d. At the end of this time, thereaction was allowed to cool and the precipitate was removed byfiltration. The filtrate was reduced to a minimal volume and to it wasadded an aqueous solution of NH₄PF₆. The resulting precipitate wascollected by filtration and washed with H₂O and ether. Collected anorange solid which was the target product in pure form. Collected 2.8 g(60%).

Example 3

MPV.2PF₆: To a solution of 4,4′-bipyridine (2 g, 12.8 mmol) in EtOH (50mL) was added 1-chloro-2,4-dinitrobenzene (1.3 g, 6.4 mmol). Thereaction was allowed to stir under reflux for 2 d. After this time, thereaction was allowed to cool to room temperature and an excess of ether(250 mL) was added to the solution. The resulting precipitate wascollected by filtration and dried under vacuum. The dried solid wasdissolved in a 70:30 EtOH/H₂O mixture along with aniline (1.3 g, 12.8mmol). The reaction mixture was heated to reflux for 3 d. After thistime, the reaction was allowed to cool to room temperature, and theprecipitate was removed by filtration. The filtrate was reduced to aminimal volume and to it was added an aqueous solution of NH₄PF₆. Theresulting precipitate was collected by filtration and washed with H₂Oand dried under vacuum. The dried solid was dissolved in MeCN (50 mL)along with MeI (1.8 g, 12.8 mmol) and the reaction was set to reflux for2 d. The solvent was reduced to a minimal volume and to it was added anaqueous solution of NH₄PF₆. The resulting precipitate was collected byfiltration and washed with H₂O and ether. Collected 2.1 g (60%) of ayellow solid which was the target product in pure form.

Example 4

MPV.2PF₆: To a solution of 4,4′-bipyridine (2 g, 12.8 mmol) in EtOH (50mL) was added 1-chloro-2,4-dinitrobenzene (1.3 g, 6.4 mmol). Thereaction was allowed to stir under reflux for 2 d. After this time, thereaction was allowed to cool to room temperature and an excess of ether(250 mL) was added to the solution. The resulting precipitate wascollected by filtration and dried under vacuum. The dried solid wasdissolved in a 70:30 EtOH/H₂O mixture along with aniline (1.3 g, 12.8mmol). The reaction mixture was heated to reflux for 3 d. After thistime, the reaction was allowed to cool to room temperature, and theprecipitate was removed by filtration. The filtrate was reduced to aminimal volume and to it was added an aqueous solution of NH₄PF₆. Theresulting precipitate was collected by filtration and washed with H₂Oand dried under vacuum. The dried solid was dissolved in MeCN (50 mL)along with MeI (1.8 g, 12.8 mmol) and the reaction was set to reflux for2 d. The solvent was reduced to a minimal volume and to it was added anaqueous solution of NH₄PF₆. The resulting precipitate was collected byfiltration and washed with H₂O and ether. Collected 2.1 g (60%) of ayellow solid which was the target product in pure form.

Example 5

R.6PF₆: T.2PF₆ (167 mg, 0.2 mmol), CBPQT.4PF₆ (110 mg, 0.1 mmol), andtris(2,2′-bipyridine)dichlororuthenium(II) hexahydrate (74.8 mg, 0.1mmol) were dissolved in MeCN (20 mL). The mixture was purged with Arwhilst stirring for 30 min, and then triethanolamine (1.49 g, 10 mmol)and di-tert-butyl acetylenedicarboxylate (226 mg, 1 mmol) were added.The reaction mixture was stirred under an Ar atmosphere in visible lightfor 3 days. The solvent was evaporated off and the residue was purifiedby column chromatography (SiO₂:MeOH and then 0.1% NH₄PF₆ in Me₂CO).Yellow fractions were collected, concentrated to a minimum volume, fromwhich the product was precipitated on addition of H₂O, before beingcollected by filtration to afford R.6PF₆ (80 mg, 35%) as a light-yellowpowder.

1.-50. (canceled)
 51. A complex comprising (a) CBPQT^(2(•+)) and (b) acompound of formula (I):

wherein the complex has a 3⁺ charge and is a tris-radical, and R¹ and R²are each independently selected from the group consisting of alkyl,cycloalkyl, heterocycloalkyl, aryl, heteroaryl, alkenyl, alkynyl,alkyleneazido, alkylenecycloalkyl, alkyleneheterocycloalkyl, andalkylenearyl, or a salt thereof.
 52. The complex of claim 51, whereinthe compound of formula (I) has a diameter up to 10 Å, about 5 Å toabout 9 Å, or about 6 Å to about 9 Å.
 53. The complex of claim 51,wherein the salt comprises an anion selected from the group consistingof PF₆ ⁻, halo, sulfate, phosphate, acetate, nitrate, trifluoroacetate,and carbonate.
 54. The complex of claim 51, wherein R¹ and R² are thesame.
 55. The complex of claim 51, wherein R¹ and R² are different. 56.The complex of claim 51, wherein R¹ or R² is methyl, ethyl, propyl,isopropyl, butyl, iso-butyl, sec-butyl, t-butyl, pentyl, hexyl, heptyl,or octyl, or R¹ or R² is phenyl, substituted phenyl, naphthyl, orsubstituted naphthyl.
 57. (canceled)
 58. The complex of claim 56,wherein the phenyl or naphthyl is substituted one or more electronwithdrawing group.
 59. (canceled)
 60. The complex of claim 56, whereinthe phenyl or naphthyl is substituted with one or more electron donatinggroup.
 61. (canceled)
 62. The complex of claim 51 in the form of acrystal.
 63. The complex of claim 62, wherein the crystal is a singlecrystalline form.
 64. The complex of claim 51, wherein R¹ or R² isfurther modified with a polymerizable group.
 65. (canceled)
 66. Apolymer comprising a complex of claim
 64. 67. A method of making acomplex of claim 51 comprising mixing CBPQT⁴⁺ and a di-cation of thecompound of formula (I) in the presence of a reducing agent to form thecomplex.
 68. The method of claim 67, wherein the reducing agent is zincdust, an electrochemical reductant, ruthenium(II)tri(2,2′-bipyridine)(Ru(bpy)₃ ²⁺), nacent hydrogen, sodium amalgam, NaBH₄, sulfitecompounds, Zn(Hg) amalgam, oxalic acid, formic acid, ascorbic acid, or ametal having a redox potential of about 0.76V to about 3.04V. 69.-73.(canceled)
 74. A method of making a crystal of claim 62 comprisingcrystallizing the complex using slow-vapor diffusion or crystallizingthe complex in the presence of an externally applied magnetic field. 75.(canceled)
 76. The method of claim 74, wherein the magnetic fieldcontrols the direction of crystal growth.
 77. The method of claim 76,wherein the magnetic field controls the crystal morphology.
 78. Themethod of claim 77, wherein the crystal morphology is one or more ofprism, pyramid, dipyramid, triganoal bipyramid, square pyramid, fiber,and nanowire. 79.-88. (canceled)
 89. The crystal of claim 62, whereinthe crystal morphology is one or more of prism, pyramid, dipyramid,triganoal bipyramid, square pyramid, fiber, and nanowire.
 90. Thecrystal of claim 62, wherein the crystal is an electrode.
 91. Thecrystal of claim 90, wherein the electrode is a component in a battery,solar cell, or charge storage device.